a(n) = (11*9^n+1)/2.

Terms

a(0) =6a(1) =50a(2) =446a(3) =4010a(4) =36086a(5) =324770a(6) =2922926a(7) =26306330a(8) =236756966a(9) =2130812690a(10) =19177314206a(11) =172595827850